1. Field of the Invention
The present invention relates generally to compact audio reproduction systems, and in particular to improving the perceived size of the sound source in compact audio reproduction systems.
2. Background Art
Conventional compact audio reproduction systems, such as televisions, shelf systems, computers, portable entertainment centers (“boom boxes”), and table radios, for example, have the general problem that they are perceived to sound “small” and at least partly as a result, such systems fail to provide a satisfying auditory experience. The perceived size of a sound source is, of course, related to the physical extent of the sound source. In addition the perceived size of a sound source depends on a number of psychoacoustic factors, many of which are poorly understood. Apparent source size has also been shown to be related to “spaciousness” or the sensation of acoustic envelopment, such as when radiating sound sources perceived to be large envelop listeners in a diffuse sound field. For example, a number of physically small sound sources widely distributed around a room may produce the impression of a large sound source by combining sound from many directions or they may create the impression of a large sound stage by creating multiple sound images around the room and a more diffuse sound field within the room. Of course, this particular manner of enveloping listeners within a diffuse sound field, giving rise to an impression of a large sound source, is not possible in a compact audio reproduction system where all of the sound sources are located in close proximity to each other. An additional dimension to the problem is that compact audio reproduction systems may be used in almost any conceivable orientation and that listeners may be almost anywhere relative to the position of the system and, further, may move about while listening.
Many different techniques have been applied to increasing the perceived size of a sound source, with varying degrees of success. One common technique has been to use two loudspeakers with a portion of the frequency range fed to one of the speakers intentionally out of phase. As is well known, out-of-phase signal components are poorly localized and tend to create the impression of a larger sound source by delocalizing the direct sound from the source. However, this technique often results in significant acoustic magnitude (frequency response) aberrations. Many listeners also perceive the “everywhere but nowhere” character of the out-of-phase signals as unpleasant.
A variation of the out-of-phase technique is the use of various combinations of so-called difference signals created by subtracting the left channel from the right channel, L-R, or vice-versa to create R-L. Difference signals generally are considered to contain proportionally greater amounts of uncorrelated ambience information. Use of difference signals to create a greater sense of ambience can be successful in creating a perception of a larger, more room filling sound but frequently at the cost of reduced intelligibility and the general perception that the sound is “less solid”. Several variations of the difference signal technique have been used and perform well in situations where the location of the listener relative to the sound sources is known. Such systems are disclosed, for example, in U.S. Pat. No. 4,748,669 to Klayman, U.S. Pat. No. 4,489,432 to Polk, and U.S. Pat. No. 4,308,423 to Cohen. However, these techniques are generally not successful for applications where the system's sound radiating elements (sound sources) are very close together and in situations where the acoustic environment of the system and location of the listeners is arbitrary.
Various other techniques have been used including multi-directional sound sources which seek to increase perceived sound source size by radiating sound in many directions. Examples of these include U.S. Pat. No. 3,104,729 to Olson and U.S. Pat. No. 3,627,948 to Nichols. Other techniques utilize a combination of reflected and direct sound such as U.S. Pat. No. 3,727,004 to Bose and early attempts to expand the perceived image of monaural systems, such as U.S. Pat. No. 2,710,662 to Camras, filed in 1946. Such techniques generally have been applied to the design of individual loudspeakers reproducing a single audio signal (channel) and intended for use in multiples, one for each signal channel, spaced widely apart, as in a stereo reproduction system or surround sound system. However, in a compact audio reproduction system the individual speakers reproducing each signal channel are typically very close to each other, often less than one foot apart. In this case, conventional multidirectional sound techniques may contribute to the impression of a larger sound source by creating a more diffuse sound field but, due to the close proximity of the sound sources to each other, they fail to preserve any sense of stereo imaging. In addition, when implemented at such a small scale, the resulting comb filtering inherent in many of these designs may lead to subjectively unacceptable levels of sound coloration. U.S. Pat. No. 3,582,553 to Bose discloses a single speaker stereo arrangement, see FIG. 7 and FIG. 9, employing multi-directional sound where most of the sound is radiated by left and right rear speakers which receive modified left and right signals, respectively. A lesser quantity of sound is radiated by front speakers which receive either a center channel signal or modified sum signal. This system avoids the problems associated with difference signals, out of phase signals and, to some extent, reduces comb filtering by maintaining a high ratio of indirect sound to direct sound. It relies on a complex pattern of reflected sounds to increase the perceived sound source size and maintain an impression of stereo imaging. Such a system may work well in certain situations which permit the system to be correctly positioned to deliver the required reflected sounds to a predetermined listening area.
A combination of the difference signal and reflected sound approach is shown in U.S. Pat. No. 3,892,624 to Shimada, where modified difference signals in opposite phase are applied to a pair of closely spaced drive units. In another embodiment, see FIG. 17 and FIG. 18, a second set of closely spaced auxiliary rear drive units receiving the same modified difference signals as their corresponding front drive units are used to generate reflected sounds for the purpose of enhancing the stereophonic effect. In a further embodiment a delay is applied to the signals reproduced by the auxiliary rear speakers for the purpose of creating an echo effect. However, as may be readily appreciated, the use of out of phase difference signals contributes to a perception that the sound is “less solid” and the use of uncompensated auxiliary rear drive units producing the same signals leads to comb filtering in addition to a perception of acoustic coloration in the reproduced sound. Introduction of a delay to these rear signals simply shifts the acoustic anomalies to lower frequencies.
U.S. Pat. No. 3,153,120 to Brown also shows the use of modified difference signals to provide stereo reproduction from a single cabinet. In one embodiment the difference signals are applied in opposite phase to a pair of closely spaced drive units facing in opposite directions which are supplemented by forward facing drive units receiving a modified sum of the two input signals. This approach suffers from the previously discussed shortcomings of both the use of difference signals and out-of-phase techniques.
So-called “virtual surround” techniques have also been used to enlarge the apparent sound source size. These techniques which utilize complex audio signal processing, attempt to create a surround sound like experience from a single pair of loudspeakers. Examples of such systems are disclosed, for example, in U.S. Pat. No. 5,799,094 to Mouri, U.S. Pat. No. 6,173,061 to Norris, and U.S. Pat. No. 5,912,976 to Klaymen. Such schemes rely on a specific relationship between the locations of the speakers and the listener, require the listener to remain in a certain location and typically require a distance between the individual speakers greater than would be practical for a compact system such as, for example, a table radio.